An internal combustion engine may utilize various types of combustion. For example, spark ignition (SI) of a homogenous mixture during the expansion stroke is one example. This method relies on a timed spark from a sparking plug in order to achieve ignition of an air/fuel mixture within the combustion chamber. Another type of combustion is homogeneous charge compression ignition (HCCI), which occurs when the temperature of the air/fuel mixture within combustion chamber attains the autoignition temperature of the specific fuel. HCCI can be used to provide greater fuel efficiency and reduced NOx production under some conditions.
In one example approach utilizing HCCI combustion, heat exchangers may be used to extract energy from the exhaust or engine coolant to heat a portion of the intake air and thereby provide heated air for attaining autoignition as utilized during the HCCI mode. In addition, unheated air may also be provided. Such a system is described in U.S. Pat. No. 6,675,579, for example.
In the above system, when the engine operates in SI mode, unheated air may be used to reduce engine knock while achieving desired engine performance. However, the inventors herein have recognized that the heated air in the hot-air passage may become stagnant and therefore gradually cool during such operation. As a result, the stagnant air in the hot-air passage may be of insufficient temperature or contain insufficient energy to achieve autoignition of the air/fuel mixture when needed. Under such conditions, when transitioning from SI mode to HCCI mode, combustion in the HCCI mode may be degraded until the “cooled” hot air is purged from the system and heated air reaches the engine. Alternatively, the HCCI mode may be delayed until such air may be purged, thus delaying the desired transition.
As such, in one embodiment, a system for an internal combustion engine, the engine having an intake and exhaust manifold, the system comprising of a heat exchanger configured to extract energy from at least a heat source that heats a first portion of intake air, a spark plug coupled to the engine, an intake passage configured to deliver said first portion of heated intake air to the engine and to deliver a second portion of intake air which bypasses said heat source, and a controller configured to direct said second portion of intake air to the engine at least when utilizing said spark plug to initiate combustion and flame propagation of an air-fuel mixture, and to at least temporarily cause said first portion of intake air to flow during said spark ignition combustion so that a temperature of said first portion of intake air is maintained above a selected value.
In this way, it is possible to maintain a ready reserve of heated air in the hot-air passage thus providing the energy necessary for attaining autoignition of the air/fuel mixture when performing a substantially rapid transition from SI mode to HCCI mode.